1. Field of the Invention
The present invention relates to a method for producing a semiconductor device.
2. Background of the Related Art
As a power semiconductor device, for example, there is a diode or an insulated gate bipolar transistor (IGBT) with a breakdown voltage of 400 V, 600 V, 1200 V, 1700 V, 3300 V, or more. The power semiconductor device is used in a power conversion device, such as a converter or an inverter. The power semiconductor device requires characteristics, such as low loss, high efficiency, a high breakdown voltage, and low costs.
FIG. 12 is a cross-sectional view illustrating a diode according to the related art. A p-type anode layer 1501 is formed on a main surface of an n− semiconductor substrate 1500 and an n+ cathode layer 1502 is formed on an opposite surface. A p-type layer which will be a termination region 1503 is formed in the outer circumference of the p-type anode layer 1501. An anode electrode 1505 is formed on the p-type anode layer 1501 and a cathode electrode 1506 is formed on a lower surface of the n+ cathode layer 1502. Reference numeral 1507 is a field plate and reference numeral 1508 is an insulating layer.
In an element, such as the diode, in order to reduce voltage oscillation which causes noise during switching, doping concentration control is required at a deep position of the n− semiconductor substrate 1500 from the front surface to the rear surface.
As a carrier concentration control method, a method has been known which generates a donor using proton implantation in which a deep range is obtained in silicon at a relatively low acceleration voltage. This method performs proton implantation for a region including a predetermined concentration of oxygen to form an n-type region. It has been known that crystal defects are generated in the silicon substrate by the proton implantation. The crystal defect is inevitable in the generation of donors and causes deterioration of electric characteristics which cause an increase in leakage current, depending on, for example, the kind or concentration of defects.
A large number of defects which are introduced by proton implantation remain in the range Rp of a proton (the distance of a position where the concentration of ions implemented by ion implantation is the highest from an implantation surface), in a proton passage region which extends from the implantation surface to the range, and in the vicinity of the implantation surface. The remaining defect is in a state close to an amorphous state since the deviation of atoms (in this case, silicon atoms) from a lattice location is large and the disorder of a crystal lattice is strong. Therefore, the remaining defect deteriorates the characteristics of the element. For example, the remaining defect becomes the scattering center of carriers, such as electrons and holes, reduce carrier mobility, and increases electric resistance. In addition, the remaining defect becomes the generation center of carriers and increases the amount of leakage current. As such, the defect which remains in the proton passage region from the implantation surface to the range of the proton by proton implantation, causes a reduction in carrier mobility and an increase in leakage current, and is strongly disturbed from a crystal state is particularly referred to as a disorder.
The disorder reduces carrier mobility and causes deterioration of characteristics, such as an increase in leakage current or conduction loss. Therefore, an appropriate crystal defect control technique is required which generates donors while suppressing an increase in leakage current.
A method of generating donors using proton implantation has been known in which one of the main donor generation factors is the substitution of hydrogen which is introduced into silicon with oxygen in a VO defect, which is a combination of a silicon vacancy and an oxygen atom, by a heat treatment and the generation of donors is accelerated by an oxygen cluster.
In the generation of the donors by proton implantation, it is effective to increase the amount of hydrogen introduced into silicon in order to increase the number of donors generated. However, when a proton dose increases, the number of crystal defects increases. In addition, when the crystal defect is recovered by a high-temperature heat treatment, the donor is vanished by the proton. Therefore, it is difficult to increase the number of donors generated due to the trade-off relationship. In order to overcome the trade-off characteristics, a method which introduces hydrogen into silicon needs to be combined with the proton implantation or the crystal defect needs to be recovered by a method other than the high-temperature heat treatment.
For example, the following techniques have been known: a technique related to a proton dose and an annealing temperature for the generation of donor by proton implantation (for example, see JP 9-260639 A, Patent Document 1); a technique in which heat treatment conditions are described for a method of generating donors using proton implantation (for example, see US 2006-0286753 A, Patent Document 2); and a technique in which the depth of a region which is formed by the method of generating donors using proton implantation from an implantation surface is described (for example, see US 2006-0081923 A, Patent Document 3).
The technique disclosed in Patent Document 1 forms a main junction in a silicon thyristor pellet, locally implants proton ions in a peripheral portion, performs a low-temperature heat treatment to locally change protons into donors, and forms a low-resistance channel stop layer. In addition, the technique forms the channel stop layer in a crystal of the crystal which is difficult to pattern, with a simple process.
The technique disclosed in Patent Document 2 relates to a method which forms a blocking zone in a semiconductor substrate. The method includes a step of preparing a semiconductor substrate which includes first and second surfaces and is doped with a first-conductivity-type base material, a step of implanting a proton into one of the first and second surfaces of the semiconductor substrate such that the proton is introduced into a first region of the semiconductor substrate which is separated from an implantation surface, and a step of performing a heat treatment which heats the semiconductor substrate for a predetermined period of time at a predetermined temperature to generate a hydrogen induced donor in both the first region and a second region which is adjacent to the first region on the implantation surface.
The technique disclosed in Patent Document 3 forms a plurality of blocking zones using proton implantation into a semiconductor substrate such that the deepest blocking zone is formed at a depth of 15 μm from an implantation surface.
However, Patent Document 1 does not disclose a detailed description of annealing in a hydrogen atmosphere. Patent Document 2 does not disclose a method of increasing the rate of change into a donor. Here, the rate of change into a donor means the percentage of the integrated concentration of a region in which a donor is generated by a proton in the depth direction with respect to a proton dose (dose). In order to increase the rate of change into a donor using proton implantation, it is necessary to introduce a large amount of hydrogen into silicon. When the proton dose increases, the number of crystal defects increases, which results in deterioration of characteristics. In addition, when the crystal defect is recovered by the high-temperature heat treatment, the donor is vanished. As such, in the generation of the donors by proton implantation, it is possible to reduce the crystal defects and to increase the rate of change into a donor.
The inventors' examination result proved that, when the range of proton implantation (the distance of a position where the concentration of ions implanted by ion implantation was the highest from the implantation surface) was greater than 15 μm as in Patent Document 3, disorder was not sufficiently reduced in the vicinity of the proton implantation surface and in the proton passage region. FIG. 13 is a characteristic diagram illustrating the comparison between carrier concentration distributions in each range when the range Rp of proton implantation is about 15 μm and is greater than 15 μm. FIG. 13(a) illustrates a case in which the range Rp is 50 μm, FIG. 13(b) illustrates a case in which the range Rp is 20 μm, and FIG. 13(c) illustrates a case in which the range Rp is 15 μm.
When the range Rp is 15 μm as illustrated in FIG. 13(c), carrier concentration in the vicinity of the proton implantation surface (the depth is 0 μm to 5 μm) and in the proton passage region is higher than the impurity concentration, 1×1014 (/cm3), of the silicon substrate and the disorder is sufficiently reduced. In contrast, when range Rp is 20 μm as illustrated in FIG. 13(b) and when the range Rp is 50 μm as illustrated in FIG. 13(a), carrier concentration in the vicinity of the proton implantation surface and in the proton passage region is greatly reduced and the disorder is not reduced. As such, when the disorder remains, the leakage current or conduction loss of the element increases. Therefore, when the range Rp of proton implantation is greater than 15 μm, a new method for reducing the disorder needs to be examined.
The invention has been made in view of the above-mentioned problems of the related art and an object of the invention is to reduce crystal defects in the generation of donors by proton implantation. In addition, an object of the invention is to improve the rate of change into a donor in the generation of donors by proton implantation.